Nuclear magnetic resonance diagnostic apparatus developed recently are constructed to phase-demodulate the detected NMR signals with two reference waves having a 90.degree. phase difference. The frequency and phase information of the resultant demodulated signals are used for image reconstruction.
Two kinds of NMR signals F'c(t) and F's(t) obtained by demodulation have relationships, which are represented by the first and second formulas, respectively, with the frequency spectrum p(.omega.) which reflects the spin density or relaxation time of the atomic nuclei. ##EQU1## where p(.omega.) is the frequency spectrum of a reception signal (which corresponds to the spin density and relaxation time of an atomic nucleus);
.omega. is angular frequency; PA0 t is time; and PA0 .DELTA..theta. is the phase difference between a reference wave and reception signal at a demodulation time.
However, if the two signals of the formulas above are Fourier transformed, p(.omega.) appears as is shown in FIG. 1(d) if Fc'(t) and Fs'(t) are as shown in FIGS. 1(b) and (c). It is impossible to obtain p(.omega.) as shown in FIG. 1(a) which is necessary to form an image, since the phase difference .DELTA..theta. exists in the first and second formulas, indicating that the phases of the reference waves and the NMR signal are not necessarily in coincidence at the demodulation time.
Consequently, in the conventional NMR apparatus, its operator has to manually adjust the phase relation between the reference wave and the NMR signal at the demodulation time to be coincident with each other, while observing the frequency spectrum after Fourier transformation. However, with such a manual adjustment, individual differences appear in the result and it takes an extremely long time for the adjustment.